Blender 2.77 no longer orbits around the z axis when using scroll wheel or numpad 6, 4?
That's a new feature:


  The View3D NumPad4/6 keys have no more a 'turntable' type of rotation, but (as all other rotating numpad keys) use a view axis (the vertical one, see T47263 and rBfe0ca82b232d).


As it caused some confusion, the developers for now reverted this new behaviour in 2.77a, so you should just update.